1. Field of the Invention
This invention pertains to a method of producing viscous oil from a formation pay zone penetrated by an injection well and a production well. The method utilizes alternating slugs of steam and hot water through the injection well into the pay zone overlying a heated flow path between the injection and production wells.
2. Description of the Prior Art
In the annotated manual of "Oil and Gas Terms," 7th Edition, by Howard R. Williams (1987) the term "Tertiary Recovery" is identified as: Enhanced recovery methods for the production of crude oil or natural gas. Enhanced recovery of crude oil requires a means for displacing oil from the reservoir rock, modifying the properties of the fluids in the reservoir and/or the reservoir rock to cause movement of crude oil in an efficient manner, and providing the energy and drive mechanism to force its flow to a production well. Chemicals or energy are injected as required for displacement and for the control of flow rate and flow pattern in the reservoir, and a fluid drive is provided to force the oil toward a production well. Basic methods include thermal methods wherein heat energy is added to the formation.
Such thermal methods have been investigated extensively as a means for recovering viscous oil from subterranean formations. The viscosity of the oil makes it essentially immobile under formation conditions, and therefore it is essentially unrecoverable by primary and secondary recovery methods. The oil typically has an API gravity of less than about 20.degree. and a viscosity of up to about 10,000 centipoise (cps) or more. The primary classes of oils meeting this standard are referred to in the industry as "heavy oils," "tar sands" and "bitumen." For example: heavy oil has a viscosity of about 100 to 10,000 cps and an API gravity of 10 to 20 whereas the tar sand oil has a viscosity of 10,000 cps or more and an API gravity of 10 or less. There are several major formations in North America (and elsewhere) that contain petroleum (oil) which has such physical properties and is too viscous to be recovered by ordinary production methods. The viscous oil reserves in Utah, California and Alberta, Canada, is reasonably estimated in the billions of barrels. See, for example, U.S. Pat. No. 4,696,345 at column 1, lines 8-14. The economic incentive to recover such reserves is huge.
Many thermal methods have been suggested as a means to recover viscous oil, and some of them have even been successful in producing oil. Some methods have proposed using slotted liners positioned in the formations as conduits for hot fluids. Others have applied heat to the formation by use of steam or hot water or by underground combustion. Many of these methods were unsuccessful because of the difficulty of forming and maintaining fluid communication between the injection well and the production well. One of the techniques used to address this communication problem has been to drill a horizontal well placed from the injection well into the pay zone and, in some instances, to the production well. Another technique utilizes the horizontal well approach and adds piping that let steam and/or hot water circulate through the piping to warm the adjacent formation. This later technique is illustrated, for example, in U.S. Pat. No. 3,994,340 and U.S. Pat. No. 4,696,345 which are incorporated by reference.
Steam flooding is another thermal method that has been used with varying degrees of success. Steam is considerably lighter than the oil and water present in the formation and thus, because of gravity segregation, it tends to rise to the top of the formation when vertical communication exists. Consequently, the injected steam channels through the top of the formation to the producing well overriding a major portion of the formation and contacting only a small fraction of the formation oil. Once steam override has begun, continued injection of steam into the formation will accomplish very little additional oil recovery. This behavior results in an inefficient oil recovery and low vertical sweep efficiency. U.S. Pat. No. 4,607,695 attempts to address this problem by injecting a mixture of steam, a noncondensable gas, and a special class of surfactants into the formation to create a "stable foam" which acts as a diverting agent to decrease the permeability of one zone (i.e., channel) and to divert steam into other portions of the formation. The present invention also addresses sweep efficiency of a steam flood, but with an entirely different approach.
Another steam flooding technique is described in U.S. Pat. No. 4,597,443. There, a predetermined amount of steam, not greater than 1.0 pore volume, is injected into the formation through an injection well at an injection rate of 4 to 7 barrels of steam (cold water equivalent) per day per acre-foot of formation and produced fluids, including oil, are recovered through a production well. The steam temperature is within the range of 500.degree. to 700.degree. F. and it has a quality of 50 to 90 percent. The high steam injection rate was said to be essential in the process to minimize heat loss to surrounding underground strata. The process also requires shutting in the injection well periodically to let the injected steam condense in the formation and let the resulting heat dissipate into the formation to reduce the viscosity of the oil. Then, a predetermined amount of hot water or low quality steam , not greater than 1.0 pore volume, is injected into the formation with no interruption of production during the steps. The process in U.S. Pat. No. 4,597,443 is "related" to the present invention in that both utilize steam and hot water in the process. The disclosure of U.S. Pat. No. 4,597,443 is incorporated by reference.
The processes in U.S. Pat. No. 4,535,845 and U.S. Pat. No. 4,037,658 are also "related" to the present invention in that both use an injection well and a producing well in a steam flood and the present method can use horizontal wells described in U.S. Pat. No. 4,535,845 and U.S. Pat. No. 4,037,658 to create the heated path between the injection and production wells. The disclosures of U.S. Pat. No. 4,535,845 and U.S. Pat. No. 4,037,658 are accordingly incorporated by reference.
Other steam flooding and thermal recovery techniques are disclosed in the following nonexhaustive list of U.S. patents: U.S. Pat. Nos. 4,515,215; 4,489,783; 4,466,485; 4,465,137; 4,460,044; 4,450,911; 4,392,530; 4,390,067; 4,303,126; 4,020,901; 3,994,340; 3,847,219; 3,682,244; 3,572,437, and references cited therein.